Rapid eutectic growth in undercooled Al-Ge alloy under free fall condition

Abstract

Eutectic growth in Al-51.6%wt Ge alloy has been investigated during free fall in a drop tube. With decreasing undercooling ΔT, the microstructural evolution has shown a transition from lamellar eutectic to anomalous eutectic. A maximum cooling rate of 4.2×104K/s and undercooling of up to 240K (0.35TE) are obtained in the experiment. The eutectic coupled zone is calculated on the basis of current eutectic and dendritic growth theories, which covers a composition range from 48%-59% Ge and leans towards the Ge-rich side. The two critical undercoolings for the eutectic transition are ΔT1*=101K and ΔT2*=178K. When ΔT⩽ΔT*1, the microstructure for Al-51.6% Ge eutectic shows lamellar eutectic. If ΔT⩾ΔT*2, the microstructure shows anomalous eutectic. In the intermediate range of ΔT*1<ΔT<ΔT*2, the microstructure is the mixture of the above two types of eutectics.